In conventional drilling a drill bit is attached to a bottom hole assembly that is connected to a drill string. Drilling is achieved by rotating the drillstring at the surface or by using a downhole motor which causes the drill bit to rotate, and together with the weight applied to the bit allows the drill to progress through the formation.
When drilling vertical wells gravity is often sufficient to provide weight to allow the drill to progress. However when lateral drilling is carried out, weight needs to be supplied to the drilling assembly downhole to progress the drilling forward.
During coil tubing (CT) and coil tubing drilling (CTD) operations, tubing is injected from the surface and pushed down through the well via an injector assembly located on the surface. Since the tubing is pushed the tubing tends to assume a helical shape in the well and eventually lock-up in the well. As a result any additional force at the surface does not translate to movement at the end of the CT, but is instead lost in friction along the length of the CT. Therefore there is a limit to the depth that the CT can reach. For example a 1.5″ diameter CT can only be pushed 3000-4000 ft laterally.
Current methods of supplying weight to the drilling assembly and conveying a drilling assembly along a well downhole include using tractor/crawler devices to increase the distance the CT could reach compared to if it was only pushed from the surface. Other methods include vibrators and lubrication agents (beads etc) in the mud; all aiming at decreasing the friction coefficient between the CT and well and thus increase the reach—or final depth the CT can achieve.
WO 2004072437 describes an apparatus that anchors to the formation when it is drilling and pulls the circulation hose and wireline cable behind it as it moves forward. A drive unit provides the weight on bit to move the drill assembly away from anchored portion and thereby drive the drill assembly forward.
These completely autonomous systems need to create the drilling torque, weight and advancement, and comprise a circulation means if required to convey the cuttings to the parent well or surface. A problem with these types of tools is any part of the tool that travels through a lateral section of a borehole is required to travel through a curve without getting stuck and must also fit in the hole drilled by the bit. The anchoring mechanisms must contend with varying formation strengths and characteristics making for more complex designs for the units. Therefore it would be beneficial if one of these functions could be removed from the cable and instead performed independently from the cable and drilling tool in the parent (vertical)—and usually much larger—well. This would enable the tool in the lateral well to be simpler, shorter and consequently cheaper and the overall LIH (Lost in Hole) cost of the operation would also decrease.
The object of the invention is to increase the lateral reach of a CT without the need to anchor the tubing and drilling tool in the anisotropic and sometimes fragile formation. In particular a downhole injector assembly is provided to supply weight downhole to a drilling assembly and to move a cable through a borehole.